The worldwide market for high power, sealed-off, CO2 lasers continues to expand because of the growing need for improvements in applications like hole drilling, marking, cutting and scribing in ceramic and other common industrial materials. Driving this market is the need for lower cost, compact lasers with output powers in the range of 500-1000 watts.
Over the past twenty years, hybrid unstable resonator technology has been shown to provide a systematic approach for enhancing the output power and improving the output mode quality of a particular class of large mode volume gas and solid state gain media lasers. This resonator technology can be generally described as having waveguide (or freespace gaussian) propagation in one transverse cavity dimension and unstable resonator propagation in a second transverse cavity dimension. Consequently, this technological approach, as exemplified in part by U.S. Pat. No. 4,719,639 (Ref. 1), U.S. Pat. No. 5,123,028 (Ref. 2) and U.S. Pat. No. 5,048,048 (Ref. 3), is ideally suited to laser gain media with a transverse cross section with a longer and a shorter side, the cross section being transverse to a cavity longitudinal axis. Such laser devices are variously described and known to those skilled in the art of high power lasers as slab or slice CO2 lasers or slab YAG lasers.
As applied to CO2 lasers, hybrid unstable resonator technology continues to exploit the early attributes of unstable resonators initially stated four decades ago in Ref 4. Namely, (1) unstable resonators can have large mode volumes even in very short resonators, (2) the unstable configuration is readily adapted to adjustable diffraction output coupling, and (3) the analysis indicates that unstable resonators should have very substantial discrimination against higher-order transverse modes. Thus, for example, lasers made according the teachings in the above-cited '028 patent achieve sealed-off CO2 output powers of over 500 W in package sizes of about 1 meter long.
However, for all of the advantages enjoyed by the technology disclosed in the above-cited '639, '048 and '028 patents, including utilization of only two all-reflecting cavity optics and the extraction of a filled in output beam, this technology can only achieve a practical variable output coupling by changing a minimum of both cavity mirror radii and the width of the mirror at the output coupling end of the resonator.
FIGS. 1 and 2 illustrate the salient features of the '639 patent technology and the '048 patent technology, respectively. FIG. 1 shows a plan view of a resonator structure that includes a conductive electrode 2 and pair of aligned reflectors 22 and 23 disposed at opposite ends of the electrode 2. Arrow 24 represents the output beam of the resonator structure. FIG. 2 shows a perspective view of a resonator structure wherein numeral 50 designates a totally reflecting concave mirror, numeral 51 designates a totally reflecting output mirror, and numeral 67 designates a discharge space corresponding to the discharge gap (height=A, width=B) between the spaced apart electrodes of the resonator structure. Numeral 511 designates a laser beam take-out means.
Because the two intra-cavity optics in a prior art high power output RF excited CO2 hybrid unstable resonator system require water cooling, they are very expensive. As a consequence, this means that the determination of the optimum output coupling for a given length and width of active medium involves performing a series of detailed and careful experiments and requires utilization of an expensive and extensive set of cavity optics. Clearly, as each new set of cavity optics for each magnification data point is placed in the resonator, the cavity has to be realigned to a precision that precludes either overcoupling or undercoupling the active medium due to the resonator being inadvertently misaligned.
Another feature of prior art unstable resonators in general, and hybrid unstable resonators in particular, to which improvements have been directed is the quality of the diffraction coupled laser output beam. In this regard, U.S. Pat. No. 5,392,309 (Ref. 5) and associated FIG. 3 teach that clipping (or shading) of the outside edge of the rectangular beam that is typically extracted just past the shorter radius of curvature cavity end mirror is a means by which the shape and also the phase front of the diffracted output beam 521 can be improved. FIG. 3 is cross section view showing a resonator structure that includes a concave totally reflecting mirror 31 and a concave take-out mirror the upper end of which is notched to form a notch 411, Numeral 5 id FIG. 3 designates a shading plate that is disposed above the notch 411. Numeral 1 designates a laser active medium that is disposed between the total reflection mirror 31 and the take-out mirror 41.
It is well appreciated by those skilled in the art of unstable resonators that higher magnification cavity designs exhibit significantly higher diffraction output coupling losses for higher order transverse modes compared to lower order transverse modes. Thus, one way to promote laser oscillation in a transverse mode with desirable output characteristics would be to choose a cavity design of higher rather than lower geometric magnification. Unfortunately, higher geometric magnification and modest to short lengths of gain media are not normally compatible because this combination will result in overcoupling the gain medium of the laser.
One approach to this dilemma is to use the teachings disclosed in U.S. Pat. No. 6,144,687 (Ref. 6), the essence of which is depicted in FIG. 4. FIG. 4 shows a resonator structure that includes a slab geometry lasing material 10 with a waveguide arrangement 12, 14 on either side thereof. Two totally reflecting mirrors 16, 18 are provided at respective ends of the waveguide and form an unstable sub-resonator. The rear mirror 16 extends over the full width of the waveguide cavity, while the front mirror 18 is of partial width, covering only one side of the resonator. Adjacent the front mirror 18 is a tuning mirror 20, at 45° to the axis of the resonator, which deflects light to exit the sub-resonator at right angles to its axis. The deflected beam then meets a partially reflective concave mirror that is external to the sub-resonator. The curvature of the external mirror 22 checks the divergence of the Intracavity resonator modes and returns light back into the waveguide. Some light 24 is coupled out of the resonator structure through the external mirror 22 and forms the output laser beam.
Early unstable resonator technology frequently employed symmetric transverse aperture gain media for CO2 lasers (Ref, 7, 8) and, therefore, the laser output was extracted in an annular form and not as a filled-in beam. It is well known to those skilled in the art of unstable resonators that the main lobe of the far field radiation pattern of a near field annular beam may contain significantly less power than the Airy function resulting from a uniformly illuminated circular aperture. Thus, some early work (Ref. 9, 10) addressed the desirability of filling in the central region of the near field annular output beam to thereby improve the far field energy distribution of the extracted laser radiation. However, since the advent of the RF pumped slab CO2 laser approach (Ref. 1, 2, 3, 5, 11, 12), which generates a large volume of gain medium between two closely spaced electrodes, the use of a one-sided hybrid unstable resonator avoids the complications associated with an annular shaped output beam because a preferred embodiment of hybrid unstable resonator laser systems yields a filled in output beam with a rectangular shape.
The appeal and utility of hybrid unstable resonator technology (Ref. 13, 14, 15, 16) is readily evident from the types of laser devices and shapes of optical media to which this technology has been applied. Furthermore, this technological approach as applied to asymmetric transverse aperture gain media clearly reflects the imprint of the early expectation (1) of above Ref. 4 with regard to large mode volumes in short resonator lengths. Likewise, with regard to expectation (3) above, it is well appreciated by those skilled in the art of unstable resonators that higher geometric magnification is associated with enhanced transverse mode discrimination. However, as a practical matter, the higher geometric magnification resonators must be rationalized with the typical case of low to moderate laser medium small signal gain. This rationalization process is especially critical in laser systems with short cavity lengths. To some extent, a higher geometric magnification hybrid unstable resonator can be made compatible with a laser system exhibiting low small signal gain by using four curved intracavity mirrors and longitudinally folding the hybrid unstable resonator (Ref. 17). In this case, taking advantage of a higher magnification resonator comes at the expense and complication associated with four curved intracavity optics. Another approach to exploring the benefits of higher geometric magnification (Ref. 6) in a hybrid unstable resonator system is to employ a combination of a totally reflecting tuning mirror and a partially transmitting mirror in a configuration that reflects a predetermined portion of the complete output beam wavefront back into the confocal region of the resonator. To realize the benefits of a higher geometric magnification in this case obviously requires the use of a very expensive, curved, partially transmitting optic and the need to use a high reflectivity intracavity beam deflection optic to direct the intracavity flux to the partial reflector.
On balance, however, for all of the advances in unstable resonator technology published and taught in the past twenty to forty years, the above expectation (2) of ref. 4 regarding the problem of easily achieving “adjustable diffraction output coupling” in hybrid unstable resonators has not been systematically addressed or solved.